1. Field of the Invention
The invention relates to a mobile communication terminal and a method of controlling power at which a base station transmits signals to a mobile communication terminal, and more particularly to a W-CDMA (Wideband Code Division Multiple Access) type mobile communication terminal and a method of controlling power at which a base station transmits signals to a W-CDMA type mobile communication terminal.
2. Description of the Related Art
Various mobile communication terminals including a W-CDMA type mobile communication terminals have been suggested as a third-generation mobile communication terminal. A system for controlling power at which a base station transmits signals to a W-CDMA type mobile communication terminal is comprised of inner-loop control in which a mobile communication terminal transmits a request to a base station from which the mobile communication directly receives signals, to increase or reduce the power at a relatively short period of time (660 microseconds), and outer-loop control in which an optimal target SIR (Signal to Interference Ratio) is determined in accordance with an average of target quality, that is, an error rate at a relatively long period of time (about 100 microseconds to a couple of seconds), as described in “Easy IMT-2000”, K. Kinoshita, 2001, published by Denki Tsushin Kyokai.
FIG. 1 is a block diagram of a section in a conventional mobile communication terminal for controlling power at which a base station transmits signals to the mobile communication terminal.
The conventional mobile communication terminal illustrated in FIG. 1 is comprised of a wireless signal transmitter/receiver 100, a de-spreader 101 which de-spreads signals having been received from a base station through the wireless signal transmitter/receiver 100, a RAKE-synthesizer 102 which RAKE-synthesizes the signals having been de-spread in the de-spreader 101, a quality detector 103 which measures quality of receives signals when a volume of received signals reaches a predetermined volume, a target quality memory 104 storing target quality therein, a quality comparator 105 which compares quality of received signals, having been measured by quality detector 103, to the target quality stored in the target quality memory 104, and produces a target SIR in accordance with the result of the comparison, a target SIR adjuster 106 which stores a target SIR transmitted from the quality comparator 105, a SIR detector 107 which measures a SIR of the received signals, a SIR comparator 108 which compares a SIR of the received signals, transmitted from the SIR detector 107, to the target SIR transmitted from the target SIR adjuster 106, and a TPC bit producer 109 which produces power-control data in accordance with the result of the comparison transmitted from the SIR comparator 108, and transmits the control data to a base station through the wireless signal transmitter/receiver 100.
The inner-loop control is accomplished as follows.
First, signals having been received from a base station through the wireless signal transmitter/receiver 100 are de-spread in the de-spreader 101. The thus de-spread signals are RAKE-synthesized in the RAKE-synthesizer 102. Then, a SIR of the received signals is measured in the SIR detector 107. The SIR comparator 108 compares a SIR of the received signal, received from the SIR detector 107, to a target SIR received from the target SIR adjuster 106. The TPC bit producer 109 produces power-control data in accordance with the result of the comparison transmitted from the SIR comparator 108, and transmits the control data to a base station through the wireless signal transmitter/receiver 100.
In the inner-loop control, when there is a difference between the target SIR and the SIR of the received signals, the mobile communication terminal transmits a request to a base station to change power at which the base station transmits signals to the mobile communication terminal. Specifically, if the SIR of the received signals is lower than the target SIR, the mobile communication terminal transmits a request to a base station to increase the power, whereas if the SIR of the received signals is higher than the target SIR, the mobile communication terminal transmits a request to a base station to reduce the power.
The outer-loop control is accomplished as follows.
The quality detector 103 measures quality of receives signals, for instance, a block error rate (BLER) when a volume of the received signals reaches a predetermined volume. Then, the quality comparator 105 compares quality of the received signals, having been measured by quality detector 103, to a target quality stored in the target quality memory 104, and produces a target SIR in accordance with the result of the comparison. The thus produced target SIR is transmitted to the target SIR adjuster 106.
In the outer-loop control, the mobile communication terminal receives target quality such as 1% of an error rate from a base station. Then, the mobile communication terminal raises a target SIR by a predetermined increment when an error rate is higher than the target quality, and reduces the target SIR by a predetermined decrement when an error rate is smaller than the target quality. The mobile communication terminal usually sets a minimum SIR such that a target SIR is not much lowered in order to avoid communication interruption caused by rapid change in communication environment.
FIG. 2 is a time chart showing how a target SIR varies in the conventional mobile communication terminal.
Hereinbelow is explained how a target SIR varies with reference to FIG. 2.
Each of P1, P2 and P3 indicates a time at which the outer-loop control is carried out. An interval among P1, P2 and P3 is expressed as T. Specifically, the quality comparator 105 compares quality of the received signals, having been measured by quality detector 103, to a target quality stored in the target quality memory 104 at every interval T.
Since the quality of the received signals is lower than the target quality (for instance, 1% of BLER) at P1 and P2, the mobile communication terminal reduces a target SIR by a predetermined decrement. The quality of the received signals reaches at a minimum SIR at P3. Thereafter, the target SIR is controlled so as not to lower beyond the minimum SIR.
Japanese Patent Application Publication No. 2003-188815 has suggested a method of controlling a target SIR to rapidly converge a target SIR to a predetermined SIR determined in relation with quality of a signal-transmission channel. In the method, after outer-loop power control starts and until BLER becomes greater than zero, a target SIR is converged into a predetermined BLER having been set in a short period of time, and after BLER became greater than zero, a target SIR is slightly varied in the vicinity of the predetermined BLER.
In the conventional outer-loop control, a target SIR is varied when a number of received blocks reaches at a predetermined number. Thus, if a volume of received data is significantly small such as packet communication, it is resulted in that a target SIR is kept high for a long period of time. That is, when BLER is calculated in the outer-loop control, it was not possible to lower a target SIR until data block sufficient for calculating target quality was received.
As a result, when a volume of received data is significantly small such as in packet communication, a base station was unnecessarily requested for a long period of time to spend power for transmitting signals. In other words, regardless of a small volume of received data, a base station was requested to spend high power for transmitting signals, resulting in increased interference with other users.
In order to solve the above-mentioned problem, the inventor suggested a mobile communication terminal and a method of controlling signal-transmission power in Japanese Patent Application No. 2004-116916, not published yet as of Sep. 1, 2005. Hereinbelow is explained the mobile communication terminal and the method of controlling signal-transmission power both suggested in Japanese Patent Application No. 2004-116916, but it should be noted that the explanation about Japanese Patent Application No. 2004-116916 is made hereinbelow only for the purpose of better understanding of the present invention. The explanation about Japanese Patent Application No. 2004-116916 does not mean that the inventor admits the Application as prior art to the present invention.
The mobile communication terminal and the method of controlling signal-transmission power make it possible to lower a target SIR by means of a counter which transmits a timing signal in accordance with which quality of received signals is measured, even when a volume of received data is small, thereby reducing power at which a base station transmits signals to the mobile station.
The suggested mobile communication terminal de-spreads signals received from a base station, RAKE-synthesizes the thus de-spread signals to measure a SIR of the received signals, compares the SIR of the received signals to a target SIR, produces power-control data in accordance with the result of the comparison, and transmits the power-control data to a base station. The mobile communication terminal includes a quality detector which measures quality of received signals when a volume of received data reaches a predetermined volume, a counter which transmits a timing signal in accordance with which quality of the received signals is measured, and a comparator which compares target quality to one of quality of the received signals having been measured by the quality detector and quality of the received signals having been measured in accordance with the timing signal, and determines a target SIR in accordance with the result of the comparison.
The mobile communication terminal having the above-mentioned structure reduces the target SIR in accordance with the timing signal transmitted from the counter, even when a volume of received data is small, ensuring that a base station is not requested to spend unnecessary power at which the base station transmits signals to the mobile station.
If communication environment of the mobile communication terminal is sufficiently good, the communication quality may be maximized in the outer-loop control, when the target SIR is lowered to a minimum target SIR. For instance, as illustrated in FIG. 2, even after the target SIR reached a minimum target SIR, BLER may become equal to 0%, in which case, since BLER is smaller than a target error rate (for instance, 1%), control for reducing power at which a base station transmits signals is not carried out. Accordingly, when communication environment of the mobile communication terminal is sufficiently good, the power at which a base station transmits signals is not reduced to a minimum, even though the power can be further reduced. As a result, the mobile communication terminal requests a base station to wastefully spend power for transmitting signals to the mobile communication terminal.
Japanese Patent Application Publication No. 2004-112097 has suggested a mobile communication terminal including a power-control module. An SIR detector measures a SIR of received signals. A SIR detector produces a TPC bit signal indicative of “up” command or “down” command in accordance with the result of the comparison of the measured SIR of received signals with a target SIR. A quality detector measures BLER. The target SIR is varied such that the measured BLER becomes close to a target BLER.